The present invention relates to a method for concurrently emitting sex pheromones of insects in the form of vapor to the atmospheric air at controlled rates or, more particularly, to a method for emitting vapors of two or more kinds of sex pheromone compounds each having activity to different species of noxious insects to the atmosphere concurrently at controlled rates.
It is widely practiced in recent years that a synthetically prepared specific chemical compound called a sex pheromone having activity to a specific species of insects is utilized in the industries of agriculture and forestry for the purpose of controlling the population of insectan pests in the fields such as farms, orchards and forests. A sex pheromone of insects is a compound having attracting activity to one of the sexes of the insectan individuals of a particular species so as to disrupt the intersexual communication and the copulation behavior of the sexes resulting in decreased rate of multiplication. It is noteworthy that the effective concentration of a sex pheromone of insects is usually extremely low in the atmospheric air.
When a sex pheromone compound is used in the field for controlling the population of a particular species of insectan pests, it is a usual practice that, instead of distributing the pheromone compound as such over the field, the pheromone compound is contained in a shaped body and the body containing or impregnated with the pheromone compound is used as a dispenser by distributing a large number of such dispensers over the field so that vapor of the pheromone compound is sustainedly released therefrom at a controlled rate over a period of time.
For example, German Patent Nos. 28 32 248 and 29 45 655 teach a dispenser of a sex pheromone compound which is a bag made of a plastic film and containing the sex pheromone compound so that the compound permeates the bag walls and the vapor thereof is released to the atmosphere. U.S. Pat. No. 4,600,146 teaches a plastic-made tubular dispenser body containing the sex pheromone compound in the capillary bore thereof so that the sex pheromone compound permeates the tube walls and is released to the atmosphere in the form of vapor off the outer surface. The tubular dispenser should preferably be bonded side-by-side in the longitudinal direction with a metal wire having plastic deformability so that the dispenser body can retain any form as bent to ensure convenience and reliability in fixedly installing the dispensers having an appropriately deformed configuration in desired positions, e.g. on the twigs in an orchard.
It is a generally undertaken practice in the prior art that a single vapor dispenser contains only a single sex pheromone compound or, if not, a mixture of a pheromone compound with one or more related compounds such as geometrical isomers of the particular pheromone compound so that it is a relatively easy matter to control the rate of vapor release from a numer of vapor dispensers.
When two species or more of insectan pests should be controlled in respective populations by using sex pheromone compounds each having activity only to one of the species, however, vapors of different pheromone compounds must be emitted to the field concurrently each at a controlled rate so that the dispensers distributed over the field should be divided into groups in such a manner that dispensers of a group contain the sex pheromone compound effective to the first species while the dispensers of the other group contain a different sex pheromone compound effective to the second species. Accordingly, the dispensers of these gorups must be prepared and distributed over the field group by group so that the overall costs for the pest control are unavoidably multiplied as a matter of course.
When a first sex pheromone compound is an aliphatically unsaturated alcohol having 10 to 12 carbon atoms while a second sex pheromone compound, the vapor of which should desirably be emitted concurrently with the first, is an aliphatically unsaturated ester or aldehyde having 14 to 20 carbon atoms, in particular, no method is known in the prior art to be deemed impossible for simultaneously controlling the rates of concurrent emission of these two types of pheromone compounds by using vapor dispensers of a single type due to the so widely different physical parameters between the compounds such as the boiling point which is much lower in the former compound than in the latter compound and the vapor pressure at room temperature which is much higher in the former compound than in the latter compound.
Various types of vapor dispensers are known in the prior art including those prepared by impregnating a porous carrier material with a sex pheromone compound and those of the spirit-lamp type in which the surface area available for the vaporization of the pheromone compound is kept constant. The mixed vapor of the different kinds of the pheromone compounds is emitted to the atmosphere without a barrier from these dispensers. Dispensers of such a barrier-free type are also not satisfactory in respect of the simultaneous control of the emission rates of the different sex pheromone compounds because liquid compounds in a mixture generally cannot be vaporized at rates proportional to their respective fractions in the mixture and the compound having a higher vapor pressure is preferentially vaporized leaving the mixture leaned of the component.
The solubility parameter denoted by a symbol .delta. is one of the physical parameters greatly differing between the above mentioned two types of the insectan sex pheromone compounds. The value of .delta. can be obtained by the method developed by Robert F. Fedors and described in Polymer Engineering and Science, volume 14 (1974), page 147 by the calculation using an equation EQU .delta.=(.SIGMA..DELTA.e.sub.i /.SIGMA..DELTA.v.sub.i).sup.1/2,
in which .DELTA.e.sub.i, given in calories/mole, is the contribution of a particular atom or atomic group to the energy of vaporization and .DELTA.v.sub.i, given in cm.sup.3 /mole, is the contribution of the particular atom or atomic group to the molar volume.
Application of the above described method to the determination of the solubility parameters .delta. of the above mentioned two types of the sex pheromone compounds gives a value of .delta.=9.9 to 10.1 for the unsaturated alcohols and a value of .delta.=8.6 to 9.0 for the unsaturated acetates and aldehydes. When these two types of the compounds having so widely different values of the solubility parameter .delta. are contained in a single dispenser having plastic-made walls in the form of, for example, tubes, bags, capsules and laminates, the rates of vapor release of the respective pheromone compounds through the plastic-made walls are greatly influenced by the value of the solubility parameter .delta.. When the dispenser walls are formed of a plastic resin having low polarity, such as polyethylenes, for example, the unsaturated acetate or aldehyde compound having 14 to 20 carbon atoms in a molecule can permeate the dispenser walls and is released to the atmosphere in the form of vapor at a controlled rate while unsturated alcohols having 10 to 12 carbon atoms in a molecule can hardly permeate the dispenser walls due to the poor affinity thereof with the polyethylene-made walls so that polyethylene-made dispensers cannot be used in the object of sustainedly releasing vapor of such an unsaturated alcohol compound as a sex pheromone of insects.
When the dispenser is formed of a plastic resin having relatively large polarity, such as an ethylene-vinyl alcohol copolymer, on the other hand, unsaturated alcohols having 10 to 12 carbon atoms in a molecule can permeate the dispenser walls at a controlled rate while dispensers made of such a plastic resin cannot be used for the purpose of sustained vapor release of the unsaturated acetates and aldehydes having 14 to 20 carbon atoms in a molecule at a controlled.
Thus, it is a generally accepted conclusion in the prior art that it would be a very difficult matter to simultaneously controlling the rates of vapor release of two different sex pheromone compounds having widely different values of solubility parameter .delta. by using a single type of a dispenser form.